The present invention relates to an improved connection between a wall for resisting lateral forces imposed on a building and the foundation of that building. Specifically, the present invention relates to a shear transfer plate that enhances the transfer of lateral shear forces acting on a wall in a light frame building from the wall into anchor bolts embedded in the concrete foundation. The shear transfer plate is preferably part of a preassembled or prefabricated unitary shearwall subcomponent of the wall, but it can be part of a shearwall that is built on site. The shearwall can be a subcomponent of the wall or it can be the entire wall, commonly created simply by sheathing the wall framing members with plywood, oriented strand board (OSB) or the like. The framing members are typically wood, but light metal framing is now fairly common in some parts of the United States. Whether a subcomponent of a wall, or the entire wall, a shearwall is designed to resist lateral forces imposed on the building such as those caused by an earthquake or by wind loading. A typical example of a preassembled or prefabricated unitary shearwall is the Simpson Strong-Tie Strong-Wall Shearwall.
The present invention improves on preceding wall designs in light frame construction by providing a new and improved method of enhancing the transfer of lateral shear forces to the anchor bolts so that intermediate foundation bolts can be eliminated. Conventionally, a sub-component wall unit has been anchored to the foundation by connecting paired anchor bolts to holdown connectors and by augmenting those anchor bolts with intermediate foundation bolts. The anchor bolts conventionally resist uplift forces and the intermediate foundation bolts conventionally resist lateral shear forces. The present invention makes it possible to use the excess strength of the anchor bolts to resist lateral shear forces as well as uplift forces.
Holdown connectors are conventionally fastened to wall studs. Wall studs that resist uplift in shearwalls are herein generally referred to as posts to differentiate them from wall studs that are not designed to resist uplift. The chords of the shear-resisting assembly are posts, rather than studs, according to this definition. These wall studs can be intermediate between the shearwall posts and they are the primary bearing members for gravity loads in exterior bearing walls of light frame buildings. The anchor bolts are embedded in the concrete with threaded ends extending up through the mudsill. The threaded ends are attached to the holdown connectors with nuts. The anchor bolts are designed to resist uplift. Between the paired anchor bolts, the shorter foundation bolts are similarly anchored in the concrete with threaded ends that extend up through the mudsill where they are attached with nuts and washers. The foundation bolts are designed to resist lateral shear forces. The present invention eliminates the need for foundation bolts by transferring lateral shear forces into the anchor bolts, thereby saving the time and expense of installing intermediate foundation bolts and also reducing the possibility of costly mistakes resulting from mis-installed foundation bolts.
Both posts and studs are typically connected to the bottom sill plate (also called the bottom strut or mudsill) via nails through the sill plate and into the end grain of the post or stud. When uplift forces act on a post or stud, these end grain nail connections offer no meaningful resistance to this uplift. Even if toe nails are used to secure the posts or stud to the bottom sill plate, these are so ineffective at resisting uplift that they are not considered as load paths for uplift forces. Uplift in posts is effectively resisted only if there is a positive mechanical restraint resisting uplift; this mechanical restraint is typically a combination of a holdown connector attached to the post and an anchor restrained by the underlying structure. Unanchored wall studs do not resist uplift unless there is some other mechanical stud-to-plate connector such as a bracket, commonly used in high wind regions. Uplift is introduced into the sill plate primarily because the sheathing is nailed to it in nailed wood structural panel shear walls. As the wall distorts horizontally, the sheathing rotation tends to cause the perimeter nails at the bottom of the sheathing to lift up on the sill plate at various locations depending on the motion of the sheathing rotation. This uplift in the sill is then usually resisted by the shear bolts along the sill. Because the uplift at the sheathing comes in at the side of the sill, and the foundation bolts are in the center of the sill, cross-grain bending of the sill is created, which stresses the wood perpendicular to the grain, and failure of the sill often occurs because wood is so weak when loaded this way. To alleviate this problem, building codes have long required heavy plate washers at foundation bolts. In essence, these plate washers effectively move the uplift resistance afforded by the foundation bolts closer to the edge of the sill plate thus reducing the buildup of cross-grain stresses. In its most preferred embodiments, the present invention does not resist the uplift on the bottom strut, thereby obviating cross grain stresses in the wood. The relatively small amount of bottom strut uplift that is created is reduced by the post-holdown combination to a level that does not cause significant separation of the shear transfer plate from the sill
The prior art includes prefabricated shearwalls that eliminate intermediate foundation bolts, but the present invention improves on these designs by providing a shear transfer plate that connects the bottom of the shearwall to the studs only in shear, and by substantially isolating tension and compression forces along the length of the bottom strut into two components, the shear transfer plate and the bottom strut, which act substantially separately, the former in tension and the latter in compression, while remaining mutually connected. Prior art templates superficially similar to the shear transfer plate of the present invention are known, but they are different in both form and function.
For instance, one prior art template is simply a bolt placement template that, like the shear transfer plate of the present invention, lies between the bottom strut of the shearwall and the foundation. It differs from the present invention in that it is not connected to the bottom surface of the bottom strut and, especially, in that it does not eliminate intermediate foundation bolts. On the contrary, its purpose is to position the anchor and foundation bolts, which pass through the template. The template is connected to the bottom strut by the intermediate foundation bolts that pass through both. This does little to enhance the shear connection between the anchor bolts and the bottom strut, and because the bottom strut is connected to the studs, it is subject to significant uplift forces. The bottom strut when lifted is clamped by the foundation bolt connections and is susceptible to breaking against them.
Another prior art template is a complex cut and bent steel member that passes under the studs, extending laterally beyond them to position anchor bolts that are fastened to the outer surfaces of the studs. This template does not pass under the bottom strut, but is instead bent up on either side of the bottom strut, being fastened to the sides of the bottom strut and extending tabs into the underlying concrete foundation along the edge of the shearwall. This template does away with intermediate foundation bolts, but instead of transferring all of the shear loads to the anchor bolts, it supplements the anchor bolts with tabs that can present relatively little shear resistance since they are oriented parallel to the shear forces they are supposed to resist. Furthermore, this template does not isolate the transfer of uplift forces between the studs and the bottom strut, since it passes under the studs and then clamps the bottom strut between its sides.
Another method of eliminating intermediate foundation bolts exists in a shearwall that connects specially formed holdown connectors to the studs above them and directly to the bottom strut by means of an integral flange that is inserted in the bottom strut and fastened to it. This method again fails to isolate the transfer of uplift forces, as it directly ties the bottom strut to the holdown connectors that terminate the lower ends of the studs.